1. Field of the Invention
Embodiments of the invention relate generally to the field of self-correcting wavelength collision avoidance. More particularly, an embodiment of the invention relates to a process of detecting wavelength collision, identifying a pair or pairs of optical network terminals (ONTs) that transmit the colliding wavelengths and collision recover where the wavelengths of the ONTs that cause collisions are re-adjusted to eliminate the collision.
2. Discussion of the Related Art
Telephone companies such as Verizon and AT&T have started to offer services over passive optical networks (PONs) using fiber-to-the-premise (FTTP) and fiber-to-the-curb (FTTC) systems such as FiOS™ and U-verse™. These systems offer dramatically higher data bandwidths by bringing optical fiber to the home or close to home. In order to maintain their upper hand in bandwidth per customer, North American cable operators have started deploying their own PON networks. These networks utilize scalable fiber-to-the-home (FTTH) systems, building upon fiber deployed to date in new builds and upgrades that can offer bandwidths similar to, or higher than, that provided by FiOS™ and U-verse™.
MSOs want to continue utilizing DOCSIS platform for wideband services such as high speed data (HSD), Voice over IP (VoIP) and other services supported by this platform, which provides for downstream data bandwidth up to 640 Mb/s or more, until such a time as yet higher data speeds are required. At such a time, the MSOs want the flexibility to upgrade their FTTH ONT device to handle Gb/s data speeds offered by passive optical networks (PONs) such as GPON or GEPON. They also want to support deployed interactive TV services that are based on set top boxes with active upstream signaling to support fully interactive services such as Video on Demand (VoD) and Switched Digital Video (SDV).
RF over Glass (RFoG) is the name given to the generic FTTH PON architecture that supports both legacy DOCSIS cable upstream signals and additional high speed (>1 Gb/s) PON service(s). FIG. 1 shows the schematic diagram of the RFoG PON architecture.
In the RFoG PON architecture, traditional cable services (analog and digital video, VOD, VoIP, HSD, etc.) are transported downstream on wavelength λd1 (typically 1550 nm), while DOCSIS cable upstream signals are on wavelength λu1 (typically 1590 nm or 1610 nm). None of these wavelengths denote a single wavelength. Rather, they denote a range of wavelengths with the nominal wavelength as listed. For example, 1310 nm wavelength commonly used for upstream signals in GEPN and GPON can encompass wavelength from 1300 nm to 1320 nm. Additional wavelengths λd2, λu2, (and possibly more wavelength pairs) are multiplexed on the same fiber using the wavelength combiner to support high-speed (Gb/s or higher) PON service(s) such as GEPON, GPON and 10 Gb/s EPON and GPON.
The downstream signal on wavelength λd1 is optically amplified in the headend/hub and broadcast to all the RFoG optical network terminals (ONTs). The upstream data on wavelength λu1 originates from cable modems attached to the ONTs on a QAM signal at some fixed RF frequency between 0-45 MHz (in North America, other sets of frequencies can be used and are used in Europe, Japan and other countries and regions). This upstream QAM signal is extracted by the band-pass filter (BPF) (optional) and fed to the cable modem termination system (CMTS) input in the headend/hub.
Although the upstream signals from all ONTs operate in the same wavelength range with the nominal wavelength (λu1) and at the same RF frequency, and are combined together by the PON splitter/combiner, wavelength collisions are avoided at the upstream optical receiver since GEPON, PON and DOCSIS systems employ time-division multiple access (TDMA). That is, the OLT or CMTS permits only one ONT or cable modem to transmit data at any given time.
The ONTs employ burst-mode transmission in the reverse path to ensure that the reverse path laser in the ONT only turns on when it is allowed to transmit (by OLT) or detects incoming data from the cable modem (that is allowed to transmit by CMTS) and is off the rest of the time. In this manner, upstream wavelength collisions are avoided. Avoiding wavelength collisions is of critical importance in a PON system—if two optical signals with the same wavelength are incident on a receiver, optical beating causes a severe degradation of the signal-to-noise ratio (SNR) over the entire return path bandwidth rendering the receiver unable to detect any signals for the duration of the wavelength collision.
A disadvantage of the conventional RFoG architecture shown in FIG. 1 is the disproportionate cost of transporting the traditional cable return signals—mainly signaling from a set-top-box (STB) and QAM channels for DOCSIS data signals. A major concern is that only one DOCSIS channel is supported in the return band (a QAM channel at a RF frequency between 0-45 MHz in North America).